The invention relates to a shaft-driven generator system with a shaft-driven generator.
A shaft-driven generator system is known from the German publication “WGA 23—ein modernes Wellengeneratorsystem (WGA 23—a modern shaft-driven generator system)”, Special Edition of the German periodical “HANSA”, Vol. 120, No. 13, 1983, July edition, P. 1203-1207. Shaft-driven generator systems are systems frequently used on ships for low-cost generation of electrical energy. The shaft-driven generator system, which is a component of an on-board power network of a ship, comprises the following components:                the shaft-driven generator driven at variable speed for generating electrical energy at variable frequency,        the frequency converter embodied as a DC-link converter for frequency-related decoupling and voltage-related decoupling of shaft-driven generator from an on-board network, consisting of generator-side rectifier, network-side inverter and intermediate DC link circuit,        the DC link choke required for DC-link converters for smoothing the DC link current,        the DC link capacitor required for DC-link converters for smoothing the DC link voltage,        the network choke for limiting short-circuit current and harmonics,        the reactive power machine with built-in run-up motor required for enhancing short-circuit protection and to meet the reactive current needs of the network,        the field rectifier if necessary with matching transformer to adjust the shaft-driven generator exciter current as part of shaft-driven generator closed-loop control,        the fully-electronic closed-loop, open-loop and monitoring system and        the battery splitter for supplying the vital functions of the shaft-driven generator system in the event of faults and short circuits.        
This shaft-driven generator system is able to be connected on the network side by means of the switch to an on-board network busbar. To securely avoid extreme loads in the network, a so-called duplex choke is provided as the network choke. By means of this duplex choke the propagation of harmonics is prevented and the short-circuit currents are significantly reduced. Duplex chokes are frequently also referred to as current divider choke coils, balancing chokes or smoothing chokes. The ideal duplex choke is a special transformer with two windings of low stray inductance on an iron core with a number of small air gaps.
The article “Netzgestaltung mit Duplexdrosseln (Designing networks with Duplex chokes)” by W. Schild and Dr. W. Planitz, printed in the German publication Jahrbuch der Schiffbautechnischen Gesellschaft (shipbuilding society yearbook), Vol. 91, 1997, P. 173 ff., discloses how a duplex choke is used for increasing protection against short circuits. In this publication two network concepts for improving the network quality on a diesel-electric-driven ferry with the aid of duplex chokes is presented. When duplex chokes are used the effects on the electrical propulsion systems are reduced. These effects on the network, which emanate from the converter of a shaft-driven generator system, generate significant harmonics in the supplying on-board network, whereby the network quality is reduced.
The service network on a ship for the supply of auxiliary units, the lighting, the nautical devices and the communication as well as the supply systems in the living area need a low-harmonic supply voltage. Exaggerated harmonic distortion can lead to additional losses, operational faults and in extreme cases to destruction of components. A sub-network with a low harmonic distortion is decoupled by means of duplex chokes.
This article also reveals that, with shaft outputs of 10 MW and more, direct converter fed synchronous motors are used as propeller drives. As well as direct converter-fed synchronous motors, converter motors with load-commutated converters are also employed as the propeller drive.
DE 10 2006 020 144 B4 discloses a method for operating a ship's drive system with a waste heat recovery as well as a ship's drive system with waste heat recovery. The ship's drive system features units such as a shaft-driven generator/motor which is supplied by means of the converter with a network-side transformer from the on-board network. The operation of the shaft-driven generator as a motor is referred to as “Power-Take-In” (PTI). The shaft-driven generator, the static frequency converter and the network-side transformer, as soon as energy is fed from the shaft-driven generator into the on-board network, likewise form a shaft-driven generator system. The operation of the shaft-driven generator as a generator, i.e. electrical energy is generated for an on-board network, is referred to as “Power-Take-Off” (PTO). In this patent however this shaft-driven generator is primarily used as booster drive alongside the diesel engine drive. This enables the main drive to be operated with favorable consumption and the diesel generator sets to be switched off.
During the period of the switchover from motor operation of the shaft-driven generator into generator operation, an energy source feeds electrical energy into the ship's network such that voltage and frequency of the ship's network do not fall below a predefined limit value. Such a switchover occurs in the event of network faults, especially a blackout of the ship. If the known shaft-driven generator system is used as a booster drive, the reactive power machine forms this energy source during the motor/generator operation switchover, from which the on-board network is supplied with energy.
In an alternative embodiment of the shaft-driven generator system a voltage source inverter is provided as a static frequency converter. This voltage source inverter has a DC link capacitor which forms the energy source, which supplies energy into the ship's network during the operational switchover of the shaft-driven generator. An especially rapid switchover is achieved if the DC link voltage converter has a self-commutated pulse-controlled converter on the generator and network side respectively, especially IGBT pulse-controlled converter.
This type of voltage source inverter with an IGBT pulse-controlled converter on the generator and network side respectively is shown in greater detail in FIG. 3 of DE 10 2005 059 760 A1. By using a voltage source inverter instead of a current source converter as drive converter in a shaft-driven generator system as booster drive, the on-board network is not only decoupled from the generator as regards frequency but also as regards voltage. In addition the voltage source inverter can provide reactive power so that a reactive power machine is no longer needed.
If a current-source converter is used as the static frequency converter of the shaft-driven generator system, in addition to a duplex choke, a transformer, a reactive power machine with run-up motor is needed. As already described at the start, to increase the short-circuit protection, a duplex choke is needed, which is ideally a special transformer. The transformer is needed for voltage matching between converter output voltage and on-board network voltage. These additional components each need installation space, which is not readily available on ships.
If a voltage source inverter were to be used as a static frequency converter of the shaft-driven generator system, a transformer would be provided for voltage matching between converter output voltage and on-board network voltage. The two converters of the voltage source inverter would have to be embodied either in two-point topology (low voltage) or three-point topology (medium voltage). For outputs of 10 MW and more a number of two-point or three-point converters would have to be connected electrically in parallel on the output side, wherein a balancing closed-loop control, known as an Δi closed-loop control, would have to be present. Semiconductor switches that can be switched off, especially IGBT (Insulated Gate Bipolar Transistor) switches of voltage class 1200V or 1700V are currently used in low-voltage converters. In medium-voltage converters on the other hand, IGBTs or IGCTs of voltage class 3300V or 4500V or 6500V are used. As the voltage class increases however the switching frequency decreases but the semiconductor price increases.
The limited switching frequencies of the IGBTs means that filter measures would have to be implemented on the on-board network side in order to comply with the system perturbation requirements of the classification societies. Particularly when medium-voltage converters are used with a DC link voltage of far beyond 1 kV, the switching frequency is restricted to a few hundred Hertz. This also makes the design of filters more difficult. The filter design is difficult right from the outset since the on-board network involves an island network which, depending on its operating state, can have different impedances and thereby different resonant frequencies. Therefore a resulting resonant frequency acting at the output of a voltage source inverter would vary, which would make matching the filter design to the switching frequency of the voltage source inverter very difficult.
The low switching frequency of the IGBTs of the generator- or network-side converter used as a static frequency converter of a shaft-driven generator system restricts the dynamics of this converter. This makes dealing with transient operating states, such as for example network short circuit, dropout and return of the network voltage, load rejection, almost impossible despite an overdimensioning of the components. In addition the filter mentioned tends to oscillate in transient operating states.
Dealing with network short circuits and other transient operating states becomes particularly important. To somehow be able to deal with these transient operating states, the voltage source inverter of a shaft-driven generator system must always remain on the network, i.e. it may not switch off because of overcurrent. This could be achieved, both in normal operation and also in the event of a short circuit, by the current being regulated by means of a two-point closed-loop control or in individual phases by firing signals being temporarily blocked and subsequently enabled again. In such cases further unpredictable harmonics in the converter output voltage would arise, which would activate any network filter present, meaning that the required limits for network harmonics could only be complied with at great expense.
For these reasons until now, especially for medium-voltage applications, no shaft-driven generator system with a DC-link converter as a static frequency converter has been constructed which completely fulfils the aforementioned requirements in the medium-voltage range.